The present invention relates generally to apparatus for transmitting signals between two relatively rotatable members, and more specifically to slip ring apparatus wherein light signals formed in a continuous ring are transmitted without physical contact between the members. The invention offers particular advantages in apparatus having a substantial number of signal channels.
Requirements to transmit electrical power and data across rotary joints have existed for many years. Traditionally, such functions were accomplished with electromechanical sliding contact slip rings. Early slip ring applications, including those for data transmission, typically involved the transmission of appreciable amounts of power. Sliding contacts were well suited to transmission of signals characterized by significant voltages and/or currents.
As the technologies involving the use of slip ring apparatus progressed, the inherent characteristics of electromechanical slip rings began to impose increasingly severe limitations on system performance. More specifically, advancing technologies required increased channel capacity combined with overall size reduction, decreased cross-coupling of signals on separate channels and decreased noise, dead band and power consumption. However, increasing the number of sliding contacts resulted in higher friction levels which increased power consumption and/or resulted in higher dead bands. Attempts to reduce friction levels by reducing contact pressure increased the susceptibility of sliding contacts to lift during vibration, thereby introducing noise and/or signal interruptions. In addition, the lower power levels of contemporary data signals were accompanied by reduced immunity to contamination, and by consequent increased likelihood of signal interruption and/or distortion.
More recently, devices utilizing light signals have been developed to overcome some of the foregoing problems. In general, such devices comprise a pair of members, one of which is rotatable with respect to the other about an axis of rotation. One of the members carries a light detector which is aligned with a light source carried by the other member. Accordingly, light signals, which may be modulated to convey data or information of interest, are transmitted across the moving/stationary interface without physical contact between relatively moving members. Typical known forms of such devices are disclosed in U.S. Pat. Nos. 3,401,232, 3,922,063 and 4,027,945 issued respectively to J. S. Goldhammer et al on Sept. 10, 1968, F. A. Marrone on Nov. 25, 1975 and M. L. Iverson on June 7, 1977. Each disclosed device employs a light source and a detector aligned with the axis of rotation. Multiple channels may be provided by arranging light sources and detectors on the axis of rotation and in concentric rings about the axis.
Such arrangements are practical and satisfactory for small numbers of channels. However, for larger numbers of channels the required area perpendicular to the axis of rotation becomes prohibitively large since the area increases approximately as the square of the number of channels. Further, where the light source and/or detector are implemented by spreading the ends of light transmitting fibers of a fiber bundle into a ring as disclosed in U.S. Pat. No. 4,027,945, the number of fibers required to implement individual channels varies approximately as the square of the radius of the rings. For larger rings, a very large number of fibers becomes necessary, and it becomes increasingly difficult to evenly distribute the light signal around the ring. Uneven distribution generally results in modulation of the transmitted light as the one member rotates relative to the other member.
The applicant has avoided problems associated with differences in channel geometry (i.e., rings of different radii) and practical limitations on the number of channels in optical slip ring apparatus by devising a configuration in which a plurality of identical channels are axially spaced along the axis of rotation. Accordingly, each channel occupies the same amount of area and volume as any other channel. The number of channels is limited only by limitations on the length of the apparatus along the axis of rotation. Further, the channels may be configured very compactly so as to avoid any requirement for prohibitively large fiber bundles, and to minimize position dependent modulation of transmitted light signals.